The invention relates to a hydroformylation process wherein CO, H2 and at least one olefin are contacted under hydroformylation conditions sufficient to form at least one aldehyde product in the presence of a catalyst.
It is well known in the art that aldehydes can be produced by reacting an olefinically unsaturated compound with carbon monoxide and hydrogen in the presence of a rhodium-organophosphite ligand complex catalyst. The product of this hydroformylation reaction process may include certain aldehyde derivatives, depending upon the reaction process. Derivatives of aldehydes include alcohols, acids, and polyols. Such aldehydes have a wide range of utility and are useful, for example, as intermediates for hydrogenation to aliphatic alcohols, for aldol condensation to produce plasticizers, and for oxidation to produce aliphatic acids.
Preferred hydroformylation processes involve continuous hydroformylation and recycling of the catalyst solution such as disclosed, for example, in U.S. Pat. Nos. 4,148,830; 4,717,775 and 4,769,498. However, notwithstanding the benefits attendant with such liquid recycle hydroformylation processes, stabilization of the catalyst and organophosphite ligand remains a primary concern of the art. Catalyst stability is a key issue in the employment of any catalyst. Loss of catalyst or catalytic activity due to undesirable reactions of the highly expensive rhodium catalyst can be detrimental to the production of the desired aldehyde. Likewise, degradation of the organophosphite ligand employed during the hydroformylation process can lead to the formation of poisoning organophosphite compounds or inhibitors or acidic by-products that can lower the activity of the rhodium catalyst. Moreover, aldehyde production costs increase when catalyst productivity decreases.
Those skilled in the art recognize that hydrolytic instability of the organophosphite ligands is a major cause of organophosphite ligand degradation and catalyst deactivation in rhodium-organophosphite ligand complex catalyzed hydroformylation processes. All organophosphites are susceptible to hydrolysis to some degree, the rate of hydrolysis of organophosphites in general being dependent on the stereochemical nature of the organophosphite. In parallel, the analogous alcoholysis reaction wherein P—OR moieties are exchanged can also significantly alter the nature of the phosphites present in the system. In general, the bulkier the steric environment around the phosphorus atom, the slower the hydrolysis rate. Moreover, all such hydrolysis reactions invariably produce phosphorus acidic compounds that catalyze the hydrolysis reactions. For example, the hydrolysis of a tertiary organophosphite produces a phosphonic acid diester, which is hydrolyzable to a phosphonic acid monoester, which in turn is hydrolyzable to H3PO3, a strong acid. Moreover, hydrolysis of the ancillary products of side reactions, such as the reaction between a phosphonic acid diester and the aldehyde, or between certain organophosphite ligands and an aldehyde, can lead to the production of undesirable strong aldehyde acids, e.g., n-C3H7CH(OH)P(O)(OH)2.
Even highly desirable sterically-hindered organobisphosphites that are not very hydrolyzable can react with the aldehyde product to form poisoning organophosphites, e.g., organomonophosphites, which are not only catalytic inhibitors, but are far more susceptible to hydrolysis and the formation of such aldehyde acid by-products, e.g., hydroxy alkyl phosphonic acids, as shown, for example, in U.S. Pat. Nos. 5,288,918 and 5,364,950. Further, the hydrolysis of organophosphite ligands may be considered as being autocatalytic in view of the production of phosphorus acidic compounds, e.g., H3PO3, aldehyde acids such as hydroxy alkyl phosphonic acids, H3PO4 and the like, and if left unchecked the catalyst system of the continuous liquid recycle hydroformylation process will become more and more acidic. Thus, in time the eventual build-up of an unacceptable amount of such phosphorus acidic materials can cause the total destruction of the organophosphite present, thereby rendering the hydroformylation catalyst totally ineffective (deactivated) and the valuable rhodium metal susceptible to loss, e.g., due to precipitation and/or deposition on the walls of the reactor.
U.S. Pat. No. 5,741,942 (col. 1) and U.S. Pat. No. 5,288,918 (col. 24) teach that one of the major problems with Rh-phosphite-based hydroformylation is the formation of alternative phosphites due to degradation of the original phosphite. The “poisoning phosphite” structure shown in '918 involves a less sterically-hindered phosphite which is capable of binding to an active Rh catalyst, rendering it inactive towards the desired hydroformylation reaction until such time that it decomposes further and the rhodium is liberated again. There are several known ways to produce this “poisoning phosphite,” which include the rhodium-catalyzed reaction shown in '918 or simple alcoholysis of the phosphite (which can be acid- or base-catalyzed). Since the desirable phosphite ligands are sterically congested, these reactions tend to be slow under the conditions present in the hydroformylation reactors. The presence of significant amounts of any poisoning phosphite is undesired in that it lowers plant efficiency.
Numerous methods have been proposed to maintain catalyst and/or organophosphite ligand stability and thereby minimize poisoning phosphite formation. For instance, U.S. Pat. No. 5,288,918 suggests employing a catalytic activity enhancing additive, such as water and/or a weakly acidic compound. U.S. Pat. No. 5,364,950 suggests adding an epoxide to stabilize the organophosphite ligand. Other factors, such as control of acidity, are important to minimize the formation of the “poisoning phosphite” or to selectively decompose any that have formed. U.S. Pat. No. 5,741,944 teaches the use of an aqueous extractor with optional amine additives to remove at least a portion of the acids as they are formed, and U.S. Pat. No. 5,763,677 teaches the use of an ion exchange resin to remove acidic species. U.S. Pat. No. 4,774,361 suggests carrying out the vaporization separation employed to recover the aldehyde product from the catalyst in the presence of an organic polymer containing polar functional groups selected from the class consisting of amide, ketone, carbamate, urea, and carbonate radicals in order to prevent and/or lessen rhodium precipitation from solution.
It is also known that phosphites decompose in the presence of alcohols, especially primary alcohols such as methanol. US 2010/0267991 (col. 4+) discloses the issue of alcoholysis of phosphites and emphasizes the importance of steric bulk about the phosphorous to minimize the hydrolysis/alcoholysis reactions. U.S. Pat. No. 6,307,110 (col. 23, ln. 55) teaches to avoid using primary alcohols with phosphite-based hydroformylation reactions. Methanol is the most reactive primary alcohol due to the lack of steric hindrance. Methanol can react with phosphites to generate methoxy-phosphite derivates, which will be substantially less sterically congested and thus highly likely to be significant hydroformylation catalyst inhibitors. U.S. Pat. No. 3,527,809 teaches that going from phenoxy- to methoxy-based phosphites reduces the ΔHNP by over 300, reinforcing the negative impact of methoxy-based phosphites. The impact of steric congestion (or ligand cone angle) is also discussed in U.S. Pat. No. 5,741,945 wherein it is taught that non-bulky monophosphite ligands cannot be used since they compete with CO, which interferes with the hydroformylation reaction.
Notwithstanding the value of the teachings of said references, the search for alternative methods and hopefully an even better and more efficient means for stabilizing the rhodium catalyst and organophosphite ligand employed remains an ongoing activity in the art. Recent advances in coal-to-chemicals technology has presented a new challenge for hydroformylation technology. Olefins that are used to produce aldehyde products traditionally have been made by cracking petroleum feedstocks, i.e., producing low molecular weight hydrocarbons from high molecular weight hydrocarbons. A new alternative source of olefins is by oxygenate conversion processes in an oxygenate-to-olefins unit (e.g., a methanol-to-olefins unit of U.S. Pat. No. 5,914,433). Methanol is used in the commercial scale preparation of olefins, such as in methanol-to-olefins processes described above as well as in purification schemes such as isobutylene removal from raffinate streams (generating MTBE, an additive used in gasoline or to regenerate isobutylene for polyisobutylene). Residual methanol from traditional hydrocarbon-based sources been kept at very low levels, e.g., <100 ppm but the newer sources of olefins may have substantially higher levels than found in traditional sources.
Industrial olefin feeds contain impurities such as sulphur, and the removal of undesirable by-products from an olefin stream can be quite difficult. For example, the removal of sulfur, nitrogen and chlorine or the removal of dimethyl ether (DME) from C4 or C5 raffinate recovered from a methyl tertiary butyl ether (MTBE) or a tertiary amyl methyl ether (TAME) unit; or the removal of oxygenate by-products, including dimethyl ether, from an oxygenate-to-olefins unit can require a significant amount of olefin feed pretreatment.
Syngas is a widely used industrial gas that comprises CO and H2. Coal is converted to syngas in plants using coal-to-chemicals technologies. The syngas is converted to other chemicals such as methanol and a variety of olefins. The syngas and the olefins are used for other downstream processes and may contain various levels of methanol due to blow-back contamination or due to vent recycle from these other processes.
Feeds with high levels of methanol may generate catalyst solutions with substantially higher levels of methanol than observed before, thereby increasing the possibility of generating methanol-based, sterically unhindered alcoholysis products which may exhibit severe reaction inhibition.
The presence of alcohols during hydroformylation in the presence of phosphines is known. EP 420 510 reports the hydroformylation of olefins in the presence of alcohols and phosphine ligands. However, phosphines are different from phosphites, in that they do not undergo hydrolysis/alcoholysis reactions. U.S. Pat. No. 4,148,830 teaches that aldehyde condensation adducts can be used as a solvent for phosphine and phosphite-based rhodium hydroformylation in long-term, continuous operation. These condensation adducts are bulky alcohols, ethers, or esters.
Diebolt et al., Advanced Synthesis & Catalysis, 354, Issue 4, March 2012, pp. 670-677, and Ali et al., J. Mol. Cat. A, 2005, 230, 9, showed that phosphites could be used in the presence of methanol in a batch-mode operation but generated acetals rather than the aldehyde. The stability of the ligands is not discussed and, in fact, the process worked well in the absence of any phosphorous ligand.
WO 2005/093010 discloses the hydroformylation of propylene containing up to 10% oxygenate contaminates including methanol (among others) using a wide variety of catalysts. No catalyst stability or long-term continuous operation data is presented.
Since ligand hydrolysis is always a concern, balancing hydrolysis vs alcoholysis rates of phosphite ligands has not been reported in the presence of a highly reactive alcohol such as methanol. In addition, the relative rates of hydrolysis of the corresponding methoxy-based poisoning phosphites has not been reported. The reactions cited in '918 are based on butanol but there are no reports of similar reactions involving methanol and there is no information regarding the potential buildup of methoxy-based phosphite inhibitors nor any means to remove them. It would be, therefore, desirable to find olefin hydroformylation methods that do not require extensive pretreatment of the olefin feed to remove contaminants.
It also would be desirable to have a hydroformylation process that could tolerate feedstocks containing higher amounts of methanol. For example, it would be desirable to have a hydroformylation process that could use olefins and/or syngas from coal-to-chemicals plants, e.g. feed streams contaminated with methanol, as this would avoid the need for extensive processing to remove residual methanol from the feed components, which removal requires additional capital expense and processing costs.